Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently-colored sub-pixels or light-emitting elements, typically red, green, and blue, to represent each image element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode displays. To present images on these displays, the display typically receives a three-color-component image input signal containing a signal for driving each pixel (i.e., pixel signals), each pixel signal including separate color-component image signals for driving the red, green, and blue sub-pixels.
Light-emitting diodes (LEDs) incorporating thin films of light-emitting materials have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 to Tang et al. shows an organic LED color display that includes an array of organic LED light-emitting elements (sub-pixels). Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, charge transport, or charge blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a pixel when a current is passed through an organic material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
These thin-film LED devices can include a patterned light-emissive layer wherein different materials are employed in the pattern to emit different colors of light when current passes through the materials. However, patterning the materials, particularly small-molecule organic LED materials, is difficult for large substrates, thereby increasing manufacturing costs. Moreover, existing patterning methods, for example employing metal shadow masks, are expensive and can damage deposited organic materials. One approach to overcoming material deposition problems on large substrates is to employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 by Cok. It is also known to employ a white-light-emitting element that does not include a color filter, for example, as taught in U.S. Pat. No. 6,919,681 by Cok et al. A design employing an unpatterned white emitter has been proposed which includes red, green, and blue color filters to form red, green, and blue sub-pixels and an unfiltered white sub-pixel to improve the efficiency of the device as taught in U.S. Patent Publication No. 2004/0113875 by Miller et al.
However, since most imaging systems provide a three-color-component image input signal to a display, it is necessary to employ a conversion method to convert an incoming three-color-component image signal to a four-or-more-color-component image signal for driving displays having sub-pixels for emitting four or more colors of light such as the display described by Miller et al referenced above. Therefore, several methods have been developed to convert a three-color-component image input signal to a four-or-more-color-component image signal suitable for driving a four-color display.
Within this art, it is recognized that there is redundancy provided by having sub-pixels for emitting more than three colors of light and that this redundancy can be used to simultaneously drive four sub-pixel colors to produce images having an increased luminance. For example, Morgan et al. in U.S. Pat. No. 6,453,067; Lee et al. in a paper entitled “TFT-LCD with RGBW Color System”, published in the 2003 Proceedings of the Society for Information Display Conference, Wang et al. in a paper entitled “Trade-off between Luminance and Color in RGBW Displays for Mobile-phone Usage”, published in the 2007 Proceedings of the Society for Information Display Conference; and Higgins in U.S. Pat. No. 7,301,543 provide methods for providing such a conversion method for liquid crystal displays having red, green, blue and white sub-pixels in which a white sub-pixel within each pixel can be driven simultaneously with red, green, and blue sub-pixels in the same pixel to produce a luminance higher than can be produced by employing only the RGB light-emitting elements to form the same chromaticity. However, each of these processing methods introduces chromatic errors. Specifically, since more luminance is added by the white sub-pixel than is removed from the red, green, and blue sub-pixels, the resulting image is desaturated. As discussed by Wang et al. (referenced above) the improved brightness with reduced color saturation can improve the image quality of some images but will degrade the quality of other images.
It should be noted, that some of these references, most notably Lee et al., mistakenly state that their algorithms do not introduce color error. However the method discussed by Lee et al. introduces very significant color errors. The method proposed by Lee et al. includes determining a minimum of the red, green, and blue color-component image input signals for each pixel from each of the red, green, and blue color-component image input signals and using this value to drive the white sub-pixel in their display. This method provides two undesirable changes to the resulting image signal. First, since the minimum of the red, green, and blue input signal values for each pixel corresponds to the neutral (white) light to be emitted by each pixel and because this value is added to the white color signal without subtracting it from the red, green, and blue color signals, each pixel will emit more white light than requested in the input signal. Secondly, the less saturated the input pixel color is (i.e., the larger the minimum of the red, green, and blue color-component image signals within each pixel signal), the more white light is added by the white channel. Therefore, the ratio of the luminance produced by each pixel to the maximum luminance for each pixel is different for each output pixel than is required to produce the image indicated by the three-color-component image input signal.
The impact of these manipulations can be determined based upon the CIE 1976 (L*a*b*) color-difference metric, which can be used to compare the perceived difference between two colors. To demonstrate the effect of the algorithm proposed by Lee et al., it is important to assume some characteristics of a display upon which the resulting image is to be produced. An OLED display will be assumed having a white-light emitter and color filters. The red, green, and blue sub-pixels will emit light having sRGB primaries with CIE 1931 x,y chromaticity coordinates of 0.64, 0.33 for red, 0.30, 0.60 for green and 0.15, 0.06 for blue. The white sub-pixel will produce D65 illumination (CIE 1931 x,y chromaticity coordinates of 0.313, 0.329). The luminance output of these emitters will be normalized such that D65 light with a peak luminance of 200 cd/sq m will be produced by either the maximum combined intensities of the red, green, and blue sub-pixels or by the maximum intensity of the white sub-pixel. To understand the impact of the algorithm proposed by Lee et al., two colors must be assumed to be displayed on both an RGB display receiving the input RGB signal and an RGBW display receiving the converted RGBW signal produced by the algorithm discussed by Lee et al. A pure yellow patch will be chosen, having red and green luminance intensity values of 1 and a blue luminance intensity of 0. The second color required is a reference white, having an input red, green, and blue intensity of 1. Using this algorithm, the converted intensities for the yellow is found by computing the minimum of 1 for red, 1 for green, and 0 for blue, which is equal to 0. Therefore the resulting output RGBW values are 1 for red, 1 for green, 0 for blue, and 0 for white. For the white color, the red, green, and blue values are all 1, the minimum of these values is 1, and therefore the white color will be rendered with 1 for red, 1 for green, 1 for blue and 1 for white. Using a typical primary matrix for the primaries shown above, the yellow patch will therefore be rendered with a sub-pixel luminance of 43 cd/m2 for red, 143 cd/m2 for green to provide a total luminance of 186 cd/m2 at x,y coordinates of 0.419, 0.505, on both the RGB and RGBW displays. White will be rendered with a luminance of 43 cd/m2 for red, 143 cd/m2 for green, 14 cd/m2 for blue for both the RGB and RGBW displays. However, based upon Lee's algorithm, the RGBW display will have an additional 200 cd/m2 of luminance, which is produced by the white sub-pixel, providing a white luminance that is twice as high for the RGBW display as was produced for the RGB display. Using the white values as the adapting or reference display color, the CIE 1976 (L*a*b*) color difference metric provides a value of 39 between the yellow color shown on the RGB and RGBW display. Since a difference in this metric value of 1 is detectable by a user for a single patch and average values on the order of 3 are detectable for a natural image, the color produced by these two displays are clearly different.
Alternative methods for converting a three-color-component image input signal to a four-or-more-color-component image input signal can also be performed in such a way as to preserve the color accuracy of an image. For example, Murdoch et al. in U.S. Pat. No. 6,897,876, Primerano et al. in U.S. Pat. No. 6,885,380, and Miller et al. in U.S. Patent Application Publication No. 2005/0212728 have discussed such methods. However, as disclosed, the peak image luminance cannot exceed the combined peak luminance of the red, green, and blue light-emitting elements at the chromaticity coordinates of the additional primary, without reducing the color saturation. Other algorithms, such as those described by Boroson et al. in U.S. Patent Application Publication No. 2007/0139437 permit the luminance produced by the white to be higher than is produced by the combined luminance of the red, green, and blue sub-pixels but reduces the relative luminance of highly saturated colors, once again reducing color fidelity.
Recently, LCD displays have been discussed having backlights with adjustable luminance. One such display is described by Brown Elliott et al. in U.S. Publication 2007/0279372. As described within this publication, an algorithm is used to analyze an input RGB image signal, dynamically adjust the luminance produced by the backlight and to convert the RGB signal to an RGBW signal, increasing a scale value within this conversion when decreasing the luminance of the backlight or decreasing a scale value within this conversion when increasing the luminance of the backlight. While this method has the potential to provide displays with a peak display luminance that is higher than the combined peak luminance of the red, green, and blue light-emitting elements without introducing larger color errors, it has at least three problems. First, it can not be implemented for emissive displays, such as organic light-emitting diode displays, since these displays have no backlight to adjust. Secondly, for these transmissive displays increasing the backlight luminance increases the inadvertent leakage of light through dark sub-pixels, reducing color accuracy in the shadow regions of the images, and third, this method requires extra hardware, which can add significant cost to the resulting display.
None of the prior-art conversion methods permit the use of four light-emitting elements to provide luminance values higher than the combined luminance red, green, and blue light-emitting elements without introducing significant color errors or adding the expense of controlling the luminance of backlights. There still remains a need, therefore, for an improved method for rendering image scenes in display devices and, in particular, for emissive displays, such as EL displays, having more than three colors of sub-pixels with a peak display luminance that is higher than the combined peak luminance of the red, green, and blue light-emitting elements, without introducing significant color errors.